Subunit structure and chromophore composition of rhodophytan phycoerythrins. Porphyridium cruentum B-phycoerythrin and b-phycoerythrin

Abstract

A comparative study is presented of the two phycoerythrins of the unicellular red alga Porphyridium cruentum. Native B-phycoerythrin has a molecular weight of 236,000 +/- 18,000 in 0.05 M potassium phosphate at pH 7.0, and an absorption spectrum with maxima at 545 nm (epsilonM = 2.41 X 10(6) M-1 cm-1) and 563 nm, and a shoulder at 498 nm. The protein carries 38 phycoerythrobilin and at least two phycourobilin prosthetic groups per 240,000 daltons. B-Phycoerythrin is composed of three dissimilar subunits, alpha and beta, each of 17,500 daltons, and gamma of 30,200 daltons. Physical, chemical, and spectroscopic data are consistent with a subunit structure (alphabeta)6gamma for B-phycoerythrin. The alpha and beta subunits carry solely phycoerythrobilin chromophores, while the gamma subunit carries both phycoerythrobilin and phycourobilin groups. The NH2-terminal sequences of the alpha and beta subunits determined by sequential Edman degradation, are shown below: alpha subunit: Met-Lys-Ser-Val-Ile-(Gly-Arg-Phe: beta subunit: Met-Leu-Asp-Ala-Phe-(Thr)-Arg-Val-Val-Val-Asn-Ala-Asx-Ala-( )-Ala-Ala-Tyr-Val. The NH2 terminus of the gamma subunit is blocked. b-Phycoerythrin is polydisperse and exhibits native molecular weights ranging from approximately 40,000 to approximately 260,000, depending on pH, ionic strength, and protein concentration. The absorption spectrum is characterized by maxima at 543 nm (epsilonM = 3.41 X 10(5) M-1 cm-1/35,000 daltons) and 563 nm. The protein carries six phycoerythrobilin groups per 35,000 daltons. b-Phycoerythrin is made up of two dissimilar types of subunits, alpha and beta, of 17,500 daltons each. The alpha and beta subunits derived from b-phycoerythrin appeared equivalent to the corresponding subunits of B-phycoerythrin on the basis of the following criteria: (a) identical chromatographic behavior on Bio-Rex 70 in acid urea; (b) similar amino acid compositions; (c) identical mobilities on polyacrylamide gels in the presence of sodium dodecyl sulfate; (d) similar phycoerythrobilin contents; (e) identical NH2-terminal sequences. These data support, but do not establish unambiguously, the conclusion that b-phycoerythrin may be a component of B-phycoerythrin. The absence or presence, and relative height, of an absorption peak (or shoulder) at 498 nm represents the major difference among the absorption spectra of different classes of phycoerythrins. The present study shows that this spectral feature is dependent on the presence and amount of phycourobilin chromophores in the native protein and is correlated with the presence of the gamma subunit.